simplify: dedupe boilerplate and remove dead code across all layers

lib/view/pages/marianum_dates/marianum_dates_view.dart

@@ -27,7 +27,7 @@ class MarianumDatesView extends StatelessWidget {
     return keys.map((key) {
       final first = byMonth[key]!.first.start;
       final label = first.formatMonthYear().toUpperCase();
-      return _MonthGroup(key: key, label: label, events: byMonth[key]!);
+      return _MonthGroup(label: label, events: byMonth[key]!);
     }).toList();
   }
 
@@ -117,8 +117,7 @@ class MarianumDatesView extends StatelessWidget {
 }
 
 class _MonthGroup {
-  final String key;
   final String label;
   final List<MarianumDate> events;
-  _MonthGroup({required this.key, required this.label, required this.events});
+  _MonthGroup({required this.label, required this.events});
 }